A Novel Design Approach for Post-Reentry Impact Survivability of Radioisotope Thermoelectric Generator Fuel

Abstract

In 1964, a U.S. Navy Transit navigation satellite powered by a SNAP-9A Radioisotope Thermoelectric Generator (RTG) failed to achieve orbit, which resulted in the reentry and burnup of the RTG in the upper atmosphere. The subsequent atmospheric dispersion of the RTG's radioactive fuel was consistent with the RTG design philosophy of the time. However, the resulting global fallout and geographical distribution of the radioactive fuel led to a change in RTG design philosophy to complete fuel containment during all accident scenarios. This philosophy change necessitated a “free release” design architecture for the radioactive fuel encapsulations from the RTG during reentry, the survival of the individual encapsulations to the thermal pulse of reentry, and maintaining their integrity upon earth impact. All subsequent RTG designs for space applications have undergone rigorous analysis and testing to ensure conformity to these requirements. However, a “free release” design architecture becomes unviable if the mass of the individual encapsulations and their reentry aeroshell assemblies, coupled with their respective drag coefficients, results in a terminal velocity at Earth impact that potentially compromises the containment boundary of the radioactive fuel. These limiting boundary conditions necessitate consideration of potential alternative design approaches. One such approach is a “controlled reentry” design architecture for the RTG and its heat source assembly. This design approach includes an integral high-drag heat shield assembly capable of absorbing significant energy during Earth impact. Discussed are concept details, risks, trades, and a path forward.